This invention relates generally to power generation systems and, more particularly, to combined cycle power plants that include large variable speed process equipment.
Some known integrated gasification combined cycle (IGCC) power plants use an air separation unit to generate a flow of oxidant for the gasification process. Generally, the air separation unit requires a source of relatively constant flow and pressure compressed air. A motor driven compressor is typically used for providing the compressed air at the necessary flow and pressure. However, the motor driven compressor is an expensive piece of equipment that uses significant electrical power. Furthermore, the motor driven compressor may be considered a parasitic load on the IGCC power plant, leading to a reduction in the overall efficiency.
Another source of air for the air processing unit is the compressor of the gas turbine associated with the IGCC power plant. However, the air provided by the compressor is at variable pressure and flows based on a load on the gas turbine.
Some known systems incorporate an apparatus that receives the air at variable pressure from the compressor and discharges the air at a nearly constant pressure. Typically, the apparatus is driven and/or energized by a turbine integrated with the IGCC power plant. This turbine may be considered a dependent energy source since the turbine is driven and/or energized by a component of the IGCC power plant. For example, but not limiting of, the turbine may comprise a steam turbine that receives steam from a heat recovery steam generator (HRSG) of the IGCC power plant. However, extracting the steam from the HRSG may not be economically preferred or commercially allowable. The operator of the IGCC power generation power plant may be able to receive higher revenue from the extracted steam. Furthermore, the operator may be contractually obligated to provide a certain amount of steam to a process.
For the foregoing reasons, there is a need for a system and method providing a source of relatively constant flow and pressure compressed air to an air processing unit. The system and method should not be a parasitic load on the IGCC power plant. The system and method should provide a source of relatively constant pressure and adjustable flow of compressed air across a load range of the gas turbine. The system and method should use an independent energy source to drive and/or energize the apparatus that receives the air at variable pressure from the compressor and discharges the air at a nearly constant pressure.